The communications industry is on the cusp of a revolution characterized by three driving forces that will forever change the communications landscape. First, deregulation has opened the local loop to competition, launching a whole new class of carriers that are spending billions to build out their networks and develop innovative new services. Second, the rapid decline in the cost of fiber optics and Ethernet equipment has made them an attractive option for access loop deployment. Third, the Internet has precipitated robust demand for broadband services, leading to an explosive growth in Internet Protocol (IP) data traffic while, at the same time, putting enormous pressure on carriers to upgrade their existing networks.
These drivers are, in turn, promoting several key market trends. In particular, the deployment of fiber optics is extending from the telecommunications backbone to Wide-Area Network(s) (WAN) and Metropolitan-Area Network(s) (MAN) and the local-loop. Concurrently, Ethernet is expanding its pervasiveness from Local-Area Network(s) to the MAN and the WAN as an uncontested standard.
The confluence of these factors is leading to a fundamental paradigm shift in the communications industry, a shift that will ultimately lead to widespread adoption of a new optical IP Ethernet architecture that combines the best of fiber optic and Ethernet technologies. This architecture is poised to become the dominant means of delivering bundled data, video, and voice services on a single platform.
Passive Optical Networks (PONs) address the “last mile” of communications infrastructure between a Service Provider's Central Office (CO), Head End (HE), or Point of Presence (POP) and business or residential customer locations. Also known as the “access network” or “local loop”, this last mile consists predominantly—in residential areas—of copper telephone wires or coaxial cable television (CATV) cables. In metropolitan areas—where there is a high concentration of business customers—the access network often includes high-capacity synchronous optical network (SONET) rings, optical T3 lines, and copper-based T1 lines.
Historically, only large enterprises can afford to pay the substantial costs associated with leasing T3 (45 Mbps) or optical carrier (OC)-3 (155 Mbps) connections. And while digital subscriber line (DSL) and coaxial cable television (CATV) technologies offer a more affordable interim solution for data, they are infirmed by their relatively limited bandwidth and reliability.
Yet even as access network improvements have remained at a relative bandwidth standstill, bandwidth has been increasing dramatically on long haul networks through the use of wavelength division multiplexing (WDM) and other technologies. Additionally, WDM technologies have penetrated metropolitan-area networks, thereby boosting their capacities dramatically. At the same time, enterprise local-area networks have moved from 10 Mbps to 100 Mbps, and soon many will utilize gigabit (1000 Mbps) Ethernet technologies. The end result is a gulf between the capacity of metro networks on one side, and end-user needs and networks on the other, with a last-mile “bottleneck” in between. Passive optical networks—and in particular EPONs—promise to break this last-mile bottleneck.
The economics of EPONs are compelling. Optical fiber is the most effective medium for transporting data, video, and voice traffic, and it offers a virtual unlimited bandwidth. But the cost of deploying fiber in a “point-to-point” arrangement from every customer location to a CO, installing active components at each endpoint, and managing the fiber connections within the CO is prohibitive. EPONs address these shortcomings of point-to-point fiber solutions by using a point-to-multipoint topology instead of point-to-point; eliminating active electronic components such as regenerators, amplifiers, and lasers from the outside plant; and by reducing the number of lasers needed at the CO.
Unlike point-to-point fiber-optic technology, which is typically optimized for metro and long haul applications, EPONs are designed to address the demands of the access network. And because they are simpler, more efficient, and less costly than alternative access solutions, EPONS finally make it cost effective for service providers to extend optical fiber into the last mile.
Accordingly, EPONs are being widely recognized as the access technology of choice for next-generation, high speed, low cost access network architectures. EPONs exhibit a shared, single fiber, point-to-multipoint passive optical topology while employing gigabit Ethernet protocol(s) to deliver up to 1 Gbps of packetized services that are well suited to carry voice, video and data traffic between a customer premises and a CO. Adding to its attractiveness, EPONs have been recently ratified by the Institute of Electrical and Electronics Engineers (IEEE) Ethernet-in-the-First Mile (EFM) task force in the IEEE 802.3ah specification.
With reference to FIG. 1, there is shown a typical EPON as part of overall network architecture 100. In particular, an EPON 110 is shown implemented as a “tree” topology between a service provider's CO 120 and customer premises 130[1] . . . 130[N], where a single trunk or “feeder” fiber 160 is split into a number of “distribution” fibers 170[1] . . . 170[N] through the effect of 1×N passive optical splitters/combiner 180.
As can be further observed with reference to this FIG. 1, the trunk fiber 160 is terminated at the CO 120 at Optical Line Terminator (OLT) device 190 and split into the number of distribution fibers 170[1] . . . 170[N] which are each either further split or terminated at an Optical Network Unit (ONU) 150[1] . . . 150[N] located at a respective customer premises 130[1] . . . 130[N].
Due to the directional properties of the optical splitter/combiner 180, the OLT 190 is able to broadcast data to all ONUs in the downstream direction. In the upstream direction, however, ONUs cannot communicate directly with one another. Instead, each ONU is able to send data only to the OLT. Thus, in the downstream direction an EPON may be viewed as a point-to-multipoint network and in the upstream direction, an EPON may be viewed as a multipoint-to-point network. In the downstream direction, the OLT functions as a transmitter and is the “point” of the point-to-multipoint network. Also in the downstream direction, the ONUs function as receivers and are the “multipoint” of the point-to-multipoint network. The ONU 150[1] . . . 150[N] is considered a multicast group. In a point-to-multipoint communications network comprising a transmitter and a plurality of receivers, wherein a single copy of a multicast message is delivered to one or more intended receivers, each receiver having associated with it an independent, pre-determined traffic profile, a method of shaping and scheduling multicast traffic at the transmitter prior to transmission to the intended receivers, said method comprising receiving, by the transmitter, a multicast packet for transmission to the intended receivers; and transmitting, the multicast packet to all of the intended receivers according to a particular one of the pre-determined traffic profile(s); said method characterized in that the traffic profile used for the multicast packet transmission is associated with only one of the one or more intended receivers.
Since multiple subscribers share the same fiber bandwidth, it is important to control and regulate the resources assigned to different services running over the shared EPON infrastructure. On some service provider networks, as few as 5% of the users have been found to consume up to 80% of the available bandwidth; on some enterprise networks, recreational activities consume up to 40% of bandwidth. The role of traffic management in these networks is essential because it enables network managers to block, limit, or guarantee bandwidth resources so that the network can support the primary business goals.
Each subscriber subscribes to the service offered by the service provider, based on the so-called service level agreement (SLA). Among other policy terms and conditions, the SLA contains the quality of service (QoS) requirements for bandwidth and delay, which is referred to as a traffic profile. The bandwidth information includes committed information rate (CIR), peek information rate (PIR), and burst size (BS). The delay information includes bounded delay and jitter. The traffic profile is used for the purpose of bandwidth allocation among subscribers in the upstream direction of the EPON system. The objective of the bandwidth allocation is to ensure that subscribers will gain fair access to the available bandwidth based on their respective traffic profiles.
The traffic profile is also used for the purpose of traffic shaping in the downstream direction of the EPON system. Generally speaking, traffic shaping is the process of delaying packets within a traffic stream to cause it to conform to some defined traffic profile. A shaper delays some or all of the packets in a traffic stream in order to bring the stream into compliance with a particular traffic profile. When implemented, a shaper usually has a finite-size buffer, and packets may be discarded if there is not sufficient buffer space to hold the delayed packets.
Reasons for the use of traffic shaping in the downstream direction at the OLT include: 1) to control access to available bandwidth; 2) to ensure that traffic to each ONU conforms to the traffic profile established for it, and 3) to regulate the flow of traffic in order to avoid congestion at the ONU that can occur when the transmitted traffic exceeds the access speed of the outgoing interface at the ONU.
Equally important, there are a number of practical considerations driving the implementation of traffic shaping in the downstream direction at the OLT. First, the packet memory at each ONU tends to be scarce, as the overall PON system cost limit its size. In addition, it is much more effective and efficient to add packet memory at the OLT since the memory can be shared by all ONUs in a dynamic manner. Adding individual memory to each ONU is not cost-effective from the standpoint of overall system cost. Second, downstream traffic is transmitted at a speed approaching 1 Gbps, while an ONU is typically connected to a physical subscriber link of only 100 Mbps. This order of magnitude difference could easily lead to packet loss if there were no traffic control in the downstream direction. Finally, traffic shaping has a positive impact on the bandwidth sharing of multiple subscribes in the downstream direction.
Unfortunately, traffic shaping of broadcast or multicast traffic in the downstream direction introduces a number of implementation issues. First, and in sharp contrast to unicast traffic, a multicast packet is sent to a number of subscribers. As a result, it is conditioned by more than one traffic shaper—generally one for each of the participating subscribers. Since this conditioning is made for each transmitted packet and requires that a comparison be made with every traffic shaper/traffic profile, it is extremely time-consuming. Unfortunately, such comparison(s) becomes unworkable as the number of subscribers to a multicast stream becomes large.
Similarly problematic for a multicast packet is the situation where one subscriber's traffic shaper may disagree or otherwise conflict with another subscriber's traffic shaper. Since only one packet is sent downstream at a time, decisions affecting multicast packet transmission or delay must be made. To ensure system integrity, such decisions must be made in a consistent manner.